Digital subscriber line (DSL) technology provides a physical layer protocol for communicating information across a twisted pair telephone line at data rates exceeding those achievable using other physical layer protocols. One form of DSL is asymmetric digital subscriber line (ADSL) communication. ADSL communication involves transmitting data in one direction (typically towards the customer premises) at a greater data rate than data is transmitted in the other direction (typically towards the local exchange). There are also other forms of DSL such as symmetric DSL (SDSL), high-speed DSL (HDSL) and very high-speed DSL (VDSL). These various forms of DSL can be referred to generally as xDSL.
xDSL communication systems are generally implemented using a digital subscriber loop access multiplexer (DSLAM) located at a central office or other local exchange termination point of the public switched telephone network (PSTN). A DSLAM typically contains a number of xDSL termination units, or modems, that can establish an xDSL link and communicate xDSL protocol data across twisted pair telephone lines. The xDSL termination units can be connected to the telephone lines via splitter devices that separate the xDSL data traffic from voice traffic on the telephone lines. A splitter is similarly used at the customer premises to separate voice and xDSL data traffic and to provide the xDSL data traffic to an xDSL termination unit located at the remote customer premises. Once established, the xDSL link allows high speed data communication between the local exchange and the customer premises.
In general, the term "customer premises equipment" (CPE) can be used to encompass the equipment located at a remote customer site in the communication system. The CPE typically includes an xDSL interface component that has an xDSL termination unit for terminating the xDSL link as well as an interface component between the xDSL termination unit and other CPE components. The xDSL interface can comprise, for example, a network interface card (NIC) that interfaces between the xDSL link and a bus on a personal computer, workstation or other computing device. The xDSL interface can also form a component of a network router or bridge, such as an Ethernet router or bridge.
There is some disagreement as to what type of data traffic should be carried on the xDSL physical layer. For example, the ADSL Forum has published two technical reports suggesting competing modes for communicating data traffic across the ADSL physical layer. One mode is an asynchronous transfer mode (ATM) in which data traffic is carried by ATM cells. The other mode is a packet mode in which data is carried using high-level data link control (HDLC) or similar framed data packets. In particular, the framed packets can include point-to-point protocol (PPP) in HDLC-like frames and frame user network interface (FUNI) frames. Because of the competing ATM and packet modes of operation for data traffic over the ADSL link, it is possible that DSLAMs implemented in different regions will use different modes. Consequently, ADSL interfaces designed to be compatible with the ATM mode of operation would not work in a packet mode region, and vice versa. Further, one of the two modes is likely to become dominant, but it is unclear which that will be.